The present invention relates to a process of vitrifying a radioactive liquid waste containing ruthenium, and more particularly to a process of vitrifying a radioactive liquid waste which can prevent the formation of gaseous ruthenium when a high-level radioactive liquid waste containing radioactive ruthenium is solidified by heat-melting the same together with a glass frit cartridge.
Conventional prior art techniques for vitrification of a high level radioactive liquid waste include a liquid-fed direct energization (e.g. Joule heated) ceramic melter system. In this system, a high-level radioactive liquid waste is fed in liquid form, after adjusting its composition if necessary, into a glass-melting furnace called a Joule heated (directly heated) ceramic melter, in which the liquid waste is subjected to evaporation drying. calcination and finally vitrification. This technique has been developed mainly in Japan West Germany, and United States of America.
Among the variations of the above described system. also known is a process which comprises absorbing a high-level radioactive liquid waste into a glass frit cartridge made of molded glass fibers, feeding the cartridge into a glass-melting furnace, in which the radioactive liquid waste is finally vitrified (see, for example, Japanese patent Laid-open Specification Nos.60-80796 and 60-186797). This process has an advantage over the case where the liquid waste is fed into the furnace without being absorbed into a glass frit cartridge in that it hardly brings about the occurrence of mist and dust and therefore can eliminate problems such as clogging of a off-gas treating apparatus.
A high-level radioactive liquid waste generated from a reprocessing of spent fuels in a light water reactor by using Purex process is a solution acidified with nitric acid and contains radioactive ruthenium which is a fission product. When vitrifying such a liquid waste in the above-described liquid-fed Joule heated ceramic melte system, radioactive ruthenium contained in the liquid waste is oxidized by a gas produced by decomposition of nitric acid or nitrate during evaporation, calcination, and vitrification of the liquid waste in the melting furnace, which brings about a phenomenon that the oxidized ruthenium is mixed in gaseous form into the off-gas. It is reported that the gaseous radioactive ruthenium contained in the off-gas in this case amounts to about 20% of the amount of the ruthenium fed into the melting furnace depending upon the conditions (see "Control of Semivolatile Radionuclides in Gaseous Effluents ai Nuclear Facilities", Technical Reports Series No.220, International Atomic Energy Agency, Vienna, 1982).
However, it is necessary from the viewpoint of environmental safety to minimize the amount of the gaseous radioactive ruthenium contained in the off-gas released into the air. For this reason, various types of wet scrubbers and adsorption columns have been developed and used for the purpose of removing the gaseous radioactive ruthenium from the off gas (see the above-described publication). Although it is technically possible to attain necessary removal capacity through combination of the above described apparatuses, it has drawbacks that the off-gas treatment system is complicated and that since a large amount of radioactive ruthenium is contained in a secondary liquid waste generated from the wet scrubbers and the like, gaseous radioactive ruthenium is again formed during the step of the treatment of such secondary liquid waste.
In order to eliminate the above-described drawbacks, an attempt has been made to decompose and remove nitric acid or nitrate (hereinafter referred to as "denitration") contained in a liquid waste with formic acid, formalin, sugar, or the like before a high level radioactive liquid waste is fed into a melting furnace, thereby suppressing the formation of gaseous radioactive ruthenium through suppression of the oxidation of ruthenium by the gas produced by decomposition of nitric acid or nitrate in the melting furnace (see, for example, N. Sasaki et al., "Solidification of the High Level Liquid Waste from the Tokai Reprocessing Plant", Proceeding of the American Nuclear Society, International Meeting on Fuel Reprocessing and Waste Management, Jackson, Wyoming, Aug. 16-29, 1984).
However, in the above-described process wherein a high-level radioactive liquid waste is treated by adding a reducing agent to the liquid waste before the liquid waste is fed into a melting furnace, nitric acid contained in the liquid waste is decomposed and removed, so that there occurs precipitation of a large amount of fission products dissolved in the liquid waste. This brings about problems that it is difficult to conduct agitation and liquid transfer and &ha& the precipitate is deposited on the inner wall of a container.